Passive geophysical prospecting apparatus and method based upon detection of discontinuities associated with extremely low frequency electromagnetic fields

ABSTRACT

An apparatus and method for passively determining the depth and thickness of a subterranean geologic formation bearing hydrocarbons, e.g., oil and/or gas, or commercially important ore deposits, e.g., precious metals. The apparatus has an antenna to pick up extremely low frequency signals naturally emanating from the Earth&#39;s surface. In a preferred embodiment, the received signal is amplified and filtered. The signal is then modulated onto a carrier wave. The modulated signal is then filtered to eliminate one of the sidebands, for example, the lower sideband. An oscillator generates a tuning frequency which is then beat against the filtered, modulated signal in order to tune to a particular frequency. The oscillator sweeps through the range of frequencies, e.g., in the upper sideband portion of the filtered, modulated signal. For each desired frequency within this range which corresponds to a certain depth in the Earth, the tuned signal is adjusted to the desired frequency and is sent to a voltage level detector for detecting the tuned information and converting same to pulses. The pulses are counted over a desired time period to determine the pulse density, i.e., number of pulses per unit of time. A counter may be used and set as a trigger to send an output signal when a specific number of pulses per unit time is achieved. The pulses or output signal may be displayed, recorded and/or sent to a computer and later manipulated by varying the specified unit of time, i.e., the time span, to examine the pulse density information.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and method for passivegeophysical prospecting. More particularly, the present inventionrelates to detecting at the Earth's surface in a non-invasive mannersubsurface discontinuities associated with extremely low frequencyelectromagnetic fields.

2. Description of Related Art

The art is replete with various passive methods and associated apparatusfor passive geophysical prospecting. There is great motivation indiscovering a reliable method of this type which is simple and thereforerelatively inexpensive when compared to actually drilling or performingnon-passive geophysical prospecting, e.g., seismic measurements and theassociated and expensive computer manipulation of resulting data, in aknown area having discontinuous strata or in an unknown area, i.e.,wildcat territory, or to determine if zones of a producing formationwere missed or just beyond the terminus of an existing well.

Several passive methods utilize an antenna to pick up naturallyoccurring frequencies emanating from the Earth's surface. Typically, thereceived signal is amplified, filtered and detected. See for exampleU.S. Pat. No. 5,148,110 to Helms which detects a time varying signalemanating from the Earth's surface. U.S. Pat. No. 4,686,475 to Kober etal. detects the vertical electric field component of telluric currentsusing a special antenna and a tunable RC filter with detection beperformed in an audio manner using the ears of an operator. This methodis subjective to the operator and therefore suffers in reliability andconsistency.

There is much noise associated with or interfering with these lowfrequency signals. This one reason why low pass and high pass filteringis employed after the initial amplification of the signal. However,because of the initial low frequencies of the signal, it is difficult todiscern the valuable information it carries and conditioning ispreferred. Those skilled in the art have been trying for a long timewithout great success to find the proper way of conditioning thereceived signal to discern this valuable information in a consistent andreliable manner. It is for this reason that such passive techniques havenot been accepted by the hydrocarbon prospecting community and relegatedto the level of "divining rods".

One problem is that simple amplification is typically not sufficient toallow the filters to operate effectively. For this reason a frequencygenerated by an oscillator has been added to the signal in certainmethods using a mixer to add amplitude and furnish a reference frequencyfor filtering the received signal. See for example, U.S. Pat. Nos.3,087,111 to Lehan et al.(amplifies signal and then adds the oscillatorfrequency), U.S. Pat. Nos. 3,197,704 to Simon et al. and 4,198,596 toWaeselynck et al. (amplifies, filters, adds oscillator frequency andthen low pass filters). For the most part, the oscillator controls thecenter pass frequency of the filters being used. However, the quality ofthe received signal is not enhanced and the problems of reliability andconsistency remain.

SUMMARY OF THE INVENTION

The present invention provides a surprisingly reliable and consistentmethod and apparatus for passive geophysical prospecting utilizing thelow frequency signals emanating from the Earth's surface. As a result ofextensive experimental testing, greater than seventy percent (70%)success rate has been experienced and reproducibility of results at agiven location achieved. This is particularly true in determining theabsence of hydrocarbons or precious metals in subsurface formations,which is quite valuable in unproven, virgin areas. The apparatus andmethod are simple and relatively inexpensive and expeditious time-wisewhen compared to methods currently being used commercially. The methodand apparatus may be used alone for hydrocarbon or precious metalprospecting or in conjunction with currently available prospectingtechniques to verify same or to identify promising and/or unpromisingareas prior to expending the effort and money to perform the moretraditional, time and labor intensive, and expensive prospectingtechniques.

Accordingly, there is provided an apparatus and method for passivelydetermining the depth and thickness of a subterranean geologic formationbearing hydrocarbons, e.g., oil and/or gas, or commercially importantores, e.g., precious metals. In one embodiment of the invention, theapparatus has an antenna to pick up an extremely low frequency signalemanating from the Earth's surface. This signal is believed to beassociated with the Earth's electromagnetic fields. Each frequency ofthis signal corresponds to a certain depth in the Earth and carriesinformation regarding the presence of such subterranean geologicformations. Unlike the prior art, the novel apparatus of the presentinvention has a product detector which receives the signal picked up bythe antenna and multiplies it with a frequency generated by anoscillator. The oscillator is capable of sweeping though the frequenciescorresponding to the depths of interest. By "beating" the frequency ofthe corresponding depth against the received signal, a tuned signal isgenerated for each tuning frequency by the product detector and containsthe information corresponding to that depth. The tuned signal may thenbe sent to a display, recorder or to a computer for later processing andevaluation.

In order to enhance the reliability and reproducibility of the apparatusaccording to the present invention, the apparatus preferably has avoltage detector. The voltage detector receives the tuned signal fromthe product detector, detects the tuned information carried by the tunedsignal and converts the tuned information to pulses. The pulses areoutputted by the voltage detector. The pulses or output signal may berecorded digitally on tape or in analog fashion on a strip recorderand/or sent to a computer for display on a CRT and later manipulatedusing methods and techniques that are well known in the art The pulsesare counted over a desired time period to determine the pulse density,i.e., number of pulses per unit of time. It is noted that the functionof the level detector may be performed by a computer acting on the tunedsignal received by it in real-time or after the fact by manipulatingstored tuned signal information and generating data equivalent ininformation content to the pulses generated by the voltage leveldetector. In either manner, the pulse density may be determined from thepulse data using the computer. Further, the pulse data may bemanipulated using the computer, e.g., by varying the specified unit oftime, i.e., the time span, to examine the pulse density information.Alternatively, with the voltage detector in place, the apparatus mayfurther comprise a counter which may be set to trigger an output signalwhen a specific number of pulses per unit time is achieved. The triggerpoint may be varied, thereby varying the specified unit of time, i.e.,the time span, to examine the pulse density information.

In view of the low frequencies and low signal strength of the signalsemanating from the Earth's surface, the received signal is preferablyconditioned to increase its frequency and signal strength to enhancedetection of the tuned signal information. Thus, in a preferredembodiment, the received signal is amplified and filtered. The signal isthen modulated onto a carrier wave. The modulated signal is thenfiltered to eliminate one of the sidebands, for example, the lowersideband. In this case, the oscillator generates a tuning frequencywhich is then beat against the filtered, modulated signal in order totune to a particular frequency. The oscillator sweeps through the rangeof frequencies in the remaining sideband portion, e.g., the uppersideband portion, of the filtered, modulated signal. For each desiredfrequency in this range, a tuned signal is generated corresponding tothe desired frequency and therefore a certain depth in the Earth. Asbefore noted in the previous embodiment, the apparatus preferably has avoltage detector. The voltage detector receives the tuned signal fromthe product detector, detects the tuned information carried by the tunedsignal and converts the tuned information to pulses. The pulses areoutputted by the voltage detector. The pulses or output signal may berecorded digitally on tape or in analog fashion on a strip recorderand/or sent to a computer for display on a CRT and later manipulated.The pulses are counted over a desired time period to determine the pulsedensity, i.e., number of pulses per unit of time. As noted above, thefunction of the level detector may be preformed by a computer acting onthe tuned signal received by it in real-time or after the fact bymanipulating stored tuned signal information and generating dataequivalent in information content to the pulses generated by the voltagelevel detector. In either manner, the pulse density may be determinedfrom the pulse data using the computer. Further, the pulse data may bemanipulated using the computer, e.g., by varying the specified unit oftime, i.e., the time span, to examine the pulse density information.Alternatively, with the voltage detector in place, the apparatus mayfurther comprise a counter which may be set to trigger an output signalwhen a specific number of pulses per unit time is achieved. The triggerpoint may be varied, thereby varying the specified unit of time, i.e.,the time span, to examine the pulse density information.

According to another embodiment of the present invention, a method forpassive geophysical prospecting is provided which comprises:

receiving a signal emanating from the Earth's surface with an antenna;

generating a received signal corresponding to the signal emanating fromthe Earth's surface;

generating a tuning frequency;

sweeping the tuning frequency at least through the range of frequenciescontained in the received signal;

multiplying the received signal and the tuning frequency to generate aproduct signal;

synchronously tuning the product signal over the range of frequenciescontained in the received signal; and

generating a tuned signal containing tuned information.

In order to enhance the reliability and reproducibility of the method ofthe present invention, the method preferably further comprises:

converting the tuned information to pulse information representativethereof. The pulse information may be manipulated using software in acomputer or hardware to determine pulse density. The pulse informationmay be generated by comparing individual pieces of information to areference point, which may also be adjusted to effect changes in thepulse density for evaluation purposes. The greater the pulse density;the greater the likelihood of a positive indication of the presence ofe.g., the desired hydrocarbon or precious metal.

In view of the low frequencies and signal strength of the signalemanating from the Earth's surface, the received signal is preferablyconditioned. Accordingly, a method for passive geophysical prospectingis provided, wherein the method comprises:

receiving a signal emanating from the Earth's surface with an antenna;

generating a received signal corresponding to the signal emanating fromthe Earth's surface;

amplifying the received signal to generate an amplified signal;

generating a carrier wave using an oscillator;

modulating the carrier wave with the amplified signal generating amodulated signal having the carrier wave, an upper sideband and a lowersideband, wherein the modulation may be either amplitude or frequencymodulation;

preferably canceling the carrier wave from the modulated signalgenerating an output signal having the upper and lower sidebands; and

eliminating one of the sidebands using a filter, preferably eliminatingthe lower sideband using a high pass filter which passes the highsideband but not the lower sideband.

The method preferably further comprises:

generating a tuning frequency;

sweeping the tuning frequency at least through the range of frequenciescontained in the filtered, modulated signal;

multiplying the filtered, modulated signal and the tuning frequency togenerate a product signal;

synchronously tuning the product signal at least over the range offrequencies contained in the filtered, modulated signal; and

generating a tuned signal for each tuning frequency, each of the tunedsignals having tuned information.

The tuning frequency is preferably generated using an oscillator whichpreferably can increment (or decrement as desired) the tuning frequencyin increments ranging from 0.01 to 10 Hz. The sweeping rate preferablyranges from 1 Hz per second to 200 Hz per second. Therefore, the methodprovides a record which relates time to the frequency tuned andaccordingly to the depth of the corresponding tuned information,typically in the form of a field discontinuity or transient occurrence,which is indicative of the presence of a hydrocarbon(s) or preciousmetals at that depth.

The product signal contains the tuned information relating to fielddiscontinuities and may be recorded either in analog or digital form forprocessing and interpretation. Analog detection of the tuned fielddiscontinuities may be performed by feeding the heterodyne productsignal to a level comparator to convert micro field pulsations, i.e.,tuned field discontinuities or transient occurrences, into, for example,5 volt representative pulses. The pulses from the comparator are countedand converted to a voltage proportionate to the number of pulses over adesired time period to drive an analog measuring device, for example, apin recorder or a rate meter. The desired time period may be from 0.01seconds to 10 seconds. Preferably, a window detector using a counter ICis used to count the number of micro pulsations within a given period oftime, i.e., a pulse density. This counter then generates a triggervoltage when the count exceeds a previously specified number within agiven time period.

These and other features and advantages of the present invention willbecome apparent from the following detailed description, whereinreference is made to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified functional representation of an embodiment of thepresent invention.

FIG. 2 is a graph depicting the relationship of depth to frequencycontained in the signal emanating from the Earth's surface.

FIG. 3 is a simplified functional representation of another embodimentof the present invention.

FIG. 4 its a simplified functional representation of another embodimentof the present invention.

FIG. 5 is a simplified functional representation of another embodimentof the present invention.

FIGS. 6A through 6G is a detailed schematic of the embodiment shown inFIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there may be seen a simplified functionalrepresentation of one embodiment of an apparatus 10 suitable for thepurposes of the present invention. The apparatus 10 has an antenna 12for receiving the low frequency signal emanating from the Earth'ssurface. The emanating signal has a frequency range in theelectromagnetic frequency range of 0 to about 5,000 Hz. Each of thefrequencies in this range is representative of subterranean informationcorresponding to a certain depth below the Earth's surface. See FIG. 2for a frequency versus depth relationship with the dots representingactual data and the dashed lines connecting the dots representing aninterpolation of the data. More particularly, the frequency to depthrelationship is dependent on the Earth's resistivity and electricalproperties for a particular area. The actual data points on FIG. 2represent an average of observed depth for the respective frequency fromseveral well known basins. As noted above, depths will vary from area toarea for each particular frequency. Though the graph in FIG. 2 may beused for estimation purposes, it is preferred that the depth/frequencyrelationship be determined for the particular area of interest. This maybe done by using the apparatus 10 to determine the locations ofsubterranean anomalies and comparing this data to information knownabout the area such as existing wells or seismic data, preferably, wherethe survey was performed using the apparatus 10. In this manner, acorrelation of frequency and depth may be established the particulararea of interest.

Referring again to FIG. 1, the antenna 12 generates a received signalrepresentative of the emanating signal and contains information for alldepths over the frequency range of the signal. The antenna 12 iselectrically connected to a product detector 14 which receives as afirst input the received signal. The product detector 14 may be ademodulator, e.g., LM1496 and LM1596 which are balanced modulatordemodulators available from National Semiconductor, Santa Clara, Calif.,which is configured as a demodulator. The apparatus 10 also includes anoscillator 16 which generates a tuning frequency. Preferably, theoscillator 16 is capable of generating tuning frequencies across therange of frequencies contained in the received signal. More preferably,the oscillator 16 is capable of sweeping through this range offrequencies, preferably in increments or decrements of from about 0.01Hz to about 10 Hz. The sweeping rate is preferably from about 1 Hz persecond to about 200 Hz per second. The oscillator 16 is electricallyconnected to the product detector 14 which receives the tuning frequencyas a second input. The product detector 14 beats the tuning frequencyagainst the received signal to generate a tuned signal for each tuningfrequency as an output. Accordingly, each tuned signal represents acertain depth within the Earth and contains information regardingsubterranean anomalies, which may be a hydrocarbon, e.g. oil and/or gas,or a mineral deposit, e.g., precious metals. The product detector 14 maybe connected to a recording device such a chart pin recorder 18 or to astorage and processing device such as a computer 20. It is noted thatthis connection need not be a direct electrical connection, but may bethrough a transmitter (not shown) which is electrically connected to theapparatus 10 which transmits the signal to a base location which has areceiver (not shown) which in turn is electrically connected to therecorder 18 and/or computer 20. The computer 20 may store the raw dataand manipulate the raw data in real-time or at a later date. As theoscillator 16 sweeps through the frequency range of the received signal,a curve is generated relative to frequency. The curve is of signalstrength at a particular depth versus frequency which represents depth.Surprisingly, the curve is for the most part reproducible in a relativesense in that the relationship of relative strength of the signal whencomparing the different frequencies appears to be reproducible. Further,the survey performed with apparatus 10 is a snap shot in time in thesense that a particular frequency is not maintained for a long period oftime to see if there is an time variation of the signal strength at theparticular location. This avoids any fluctuations in the overallstrength of the received signal which may introduce unnecessary errorinto the survey.

Accordingly, the survey performed by the apparatus and obtained by themethod of the present invention is distinguishable from that disclosedin U.S. Pat. No. 5,148,110 to Helms. Further, the survey of the presentinvention is conducted while stationary. Helms recognizes that therewill be no change in the received signal, but is not interested in thesignal content to determine the character of the subterranean formation.Helms uses his invention in a stationary setting only to detect a timerate of change which occurs when a transient anomaly occurs in thevicinity of his detector, for example, when seismic activity occurs orpassage of a large conducting or nonconducting mass passes past thedetector, e.g., a submarine. See column 7, line 63- column 8, line 15thereof. This is what Helms refers to a time varying signal. Otherwise,Helms uses his invention while traversing the surface of the Earth todetect changes at specific frequencies as a function of distancetraversed to locate subterranean anomalies. This is what Helms refers toas a (surface) location varying signal.

Referring now to FIG. 3, there is shown a functional representationanother embodiment of the present invention as apparatus 100. In orderto enhance the reliability and reproducibility of the apparatusaccording to the present invention, the apparatus 100 has a voltagedetector 22. The apparatus 100 is like apparatus 10, with the exceptionthe apparatus 100 also has the voltage detector 22 and optionally alsohas a counter 24 (shown in dashed lines). The voltage detector 22receives the tuned signal from the product detector 14, detects thetuned information carried by the tuned signal and converts the tunedinformation to pulses. The pulses are outputted by the voltage detector22. The pulses or output signal may be recorded digitally on tape or inanalog fashion on a strip recorder 18 and/or sent to a computer 20 fordisplay on a CRT (not shown) and later manipulated. It is noted that thefunction of the voltage level detector 22 may be performed by thecomputer 20 in apparatus 10 (FIG. 1) by acting on the tuned signalreceived by it in real-time or after the fact by manipulating storedtuned signal information and generating data equivalent in informationcontent to the pulses generated by the voltage level detector 22.

Preferably, the pulses are counted over a desired time period todetermine the pulse density, i.e., number of pulses per unit of time.Whether the computer 20 in apparatus 10 or the voltage level detector 22in apparatus 100 generates the pulse data, the pulse density may bedetermined from the pulse data using the computer 20. Further, the pulsedata may be manipulated using the computer 20, e.g., by varying thespecified unit of time, i.e., the time span, to examine the pulsedensity information. Alternatively, as shown in FIG. 3 with the voltagelevel detector 22 in place, the apparatus 100 may further comprise acounter 24 (shown in dashed lines) which may be set to trigger an outputsignal when a specific number of pulses per unit time is achieved. Thetrigger point may be varied, thereby varying the specified unit of time,i.e., the time span, to examine the pulse density information. Theoutput of the voltage level detector 22 would be received by the counter24 rather than by the recorder 18 and the computer 20. Alternatively, ifdesired, the recorder 18 and computer 20 could receive the outputs fromboth the voltage level detector 22 and the counter 24.

Referring now to FIG. 4, there is shown a simplified functionalrepresentation of another embodiment of the present invention. In viewof the low frequencies and low signal strength of the signals emanatingfrom the Earth's surface, the received signal is preferably conditionedto increase its frequency and signal strength to enhance detection ofthe tuned signal information. Thus, in FIG. 4, there is shown apparatus200 having an antenna 12 for receiving the signal emanating from theEarth's surface and generating a received signal. The antenna 12 iselectrically connected to an amplifier 26 which in turn is electricallyconnected to a filter 28. The amplifier 26 may be, for example, a TL072dual operational amplifier available from Texas Instruments.Accordingly, the received signal is amplified and filtered resulting inan amplified, filtered signal. The filter 28 is electrically connectedto a modulator 30. The modulator 30 is preferably a sideband modulator,e.g., LM1496 and LM1596 which are balanced modulator demodulatorsavailable from National Semiconductor, Santa Clara, Calif. The modulator30 is also electrically connected to an oscillator 32 which generates acarrier wave. The oscillator 32 may be any sine wave oscillator. Thesignal is then modulated onto the carrier wave generating a modulatedsignal. The modulated signal is then filtered by filter 34 to eliminateone of the sidebands, for example, the lower sideband which is themirror image of the upper sideband and therefore contains redundantinformation relative to the upper sideband. The modulator 30 may also beconfigured and used as a single sideband suppressed carrier demodulatorin which the resulting signal only has one of the sidebands. Theresulting signal may be used in other prior art passive geophysicalprospecting devices such as those identified in the background sectionhereof since the resulting signal has been strengthened power-wise andfiltered and accordingly has a better signal to noise ratio than signalswhich had been conditioned by prior art methods.

However, as shown in FIG. 4, the apparatus 200 preferably has a productdetector 14, an oscillator 16, and a recorder 18 and computer 20. Inthis case, the oscillator 16 generates a tuning frequency which is thenbeat against the filtered, modulated signal by the product detector 14in order to tune to a particular frequency. The oscillator 16 sweepsthrough the range of frequencies in the upper sideband portion of thefiltered, modulated signal. For each desired frequency in this range, atuned signal is generated corresponding to the desired frequency andtherefore a certain depth in the Earth. The output of the productdetector 14 is then sent to the recorder 18 and/or the computer 20.Alternatively (not shown), the output of the product detector 14 may besent to a voltage detector 22 and then to the recorder 18 and/or thecomputer 20 as shown in FIG. 3. As a further alternative, the output ofthe voltage level detector 22 may be sent to a counter 24 and then tothe recorder 18 and/or the computer 20 again as shown in FIG. 3 (usingthe dashed line configuration).

As discussed in regards to FIG. 3, if the apparatus 200 has a voltagedetector 22, the voltage detector 22 receives the tuned signal from theproduct detector 14, detects the tuned information carried by the tunedsignal and converts the tuned information to pulses. The pulses areoutputted by the voltage detector 22. The pulses or output signal may berecorded digitally on tape or in analog fashion on a strip recorder 18and/or sent to a computer 20 for display on a CRT (not shown) and latermanipulated The pulses are counted over a desired time period todetermine the pulse density, i.e., number of pulses per unit of time. Asnoted above, the function of the voltage level detector 22 may bepreformed by a computer 20 using methods and techniques known to thoseskilled in the art by acting on the tuned signal received by it inreal-time or after the fact by manipulating stored tuned signalinformation and generating data equivalent in information content to thepulses generated by the voltage level detector 22. In either manner, thepulse density may be determined from the pulse data using the computer20. Further, the pulse data may be manipulated using the computer 20,e.g., by varying the specified unit of time, i.e., the time span, toexamine the pulse density information. Alternatively, with the voltagelevel detector 22 in place similar to FIG. 3, the apparatus 200 mayfurther comprise a counter 24 which may be set to trigger an outputsignal when a specific number of pulses per unit time is achieved. Thetrigger point may be varied, thereby varying the specified unit of time,i.e., the time span, to examine the pulse density information.

Now referring to FIG. 5, there is shown a functional representation ofanother embodiment of the present invention. An apparatus 300 isdepicted having an antenna 12, a first amplifier 26a, a first high passfilter 28a, a low pass filter 28b, a second amplifier 26b, a modulator30, an oscillator 32, a second high pass filter 34, a demodulatorconfigured as a synchronous product detector 14, an oscillator 16, a lowpass filter 44, a third amplifier 36, a meter 38, a voltage leveldetector 22, a rate meter 40 and a pattern detector 42.

The antenna 12 in one embodiment is metal core made of a metal which hasa high susceptibility to magnetic fields but low in retention time. Themetal may be ,e.g., Permalloy. The metal core is then wound with, e.g.,copper magnet wire. The assembly is then placed within a protectivesheath, e.g., a PVC tube. In a particular embodiment, the Permalloy corewas about 3/8 inch in diameter about 4 to 6 inches long. a single layerof 16 gauge copper magnet wire was wound around the circumference of themetal core over about 2/3 of the length of the core. By having this typeof antenna relatively short in core length, the creation of inductanceis minimized therefore preserving the received signal. In use, theantenna may be laid on the surface of the ground with the main axis ofthe antenna substantially parallel thereto, or more specifically,substantially perpendicular to an imaginary ray extending from thecenter of the Earth to the surface of the Earth.

In a specific embodiment, the received signal was then amplified byamplifier 26a using a TL072 dual operational amplifier. The high passfilter 28a was an active high pass filter consisting of a 9-pole, -120db drop at 60 Hz using a TL072 dual operational amplifier. The low passfilter 28b consisted of a 2 pole -60 db drop at 8,000 Hz using a TL072dual operational amplifier. The combination of the high pass filter 28aand low pass filter 28b resulted in a pass band of frequencies fromabout 90 Hz to about 8,000 Hz. The band passed signal is the amplifiedusing amplifier 26b and sent to the modulating input of the sidebandmodulator 30 which was an LM1496 balanced modulator demodulator. Thecarrier wave was supplied using a sine wave oscillator 32 set at 20,000Hz. The modulated signal is high pass filtered using a 2 pole -40 dbdrop at 20,000 Hz active filter 34 comprising an LM356 operationalamplifier. The resulting output has a frequency bandwidth of from about20,000 Hz to about 28,000 Hz, which was the upper sideband of themodulated signal. This signal is then sent to the signal input of aNational Semiconductor LM1496 balanced modulator demodulator configuredas a synchronous product detector 14 outputting the product of theinputted signal and the output signal of the tuning oscillator 16. Thetuning sweep oscillator 16 generated a sine wave of from 20,000 Hz to25,000 Hz. The sweep oscillator 16 also had the ability to continuouslysweep through the bandwidth of from 20,000 Hz to 25,000 Hz at variousset continuous sweep rates of from 1 Hz per second to 50 Hz per second.The detected signal resulting from the product detector 14 was then sentto a passive low pass filter 44 set at 10,000 Hz, and was then amplifiedusing a National Semiconductor LM380 audio amplifier 36. The output ofthe audio amplifier 36 contained the demodulated, valuable informationfor the frequency tuned. This output signal was then sent to the voltagelevel detector 22 comprising a National Semiconductor LM311 voltagecomparator. This detector 22 compared the signal input thereto against aselectable DC voltage to detect scalar voltage potentials existing inthe tuned demodulated signal. The reference DC voltage level may beadjusted using a potentiometer to a desired level to increase or reducethe sensitivity of the detector 22. For example, the reference DCvoltage level could be set to a value such as 4.95 volts so that slightvariations above this level will be recognized in the signal range ofinterest. The comparator, i.e., detector, 22 was configured to outputpulses of from 0 to 5 volts representing the detected scalar voltagesthat represent the important information about subterranean geologicformations and their contents, i.e., hydrocarbons or precious metals.The output of the comparator 22 was sent to a rate meter 40 to beconverted from pulses per second to a corresponding voltage. The voltageoutput of the rate meter 40 was used to establish a base line referencefor recording purposes. The output of the comparator 22 was also sent toa pattern detector 42 which counted the number of pulses in a givenperiod of time and outputs a response to a recorder 18 (not shown inFIG. 5) when a preselected number of pulses for a given time period wasencountered or exceeded. The preselected number of pulses in a giventime period is preferably adjustable. This variable may be adjustedbased on the activity encountered in the signal of interest. Thisdifference in activity may be due to the difference in material beingprospected, e.g., oil versus gold, and/or the quantity of such materialencountered in the subterranean formation. An output from the comparator22 was also available to a computer 20 (not shown on FIG. 5) where thepulses are digitized and processed using methods and techniques known bythose skilled in the art to determine the pulse density over a selectedperiod or unit of time. The processed information may then be printedusing a printer (not shown).

FIGS. 6A-6D with minor differences (detailed in parentheticals whereappropriate) are a detailed schematic of the embodiment shown in FIG. 5.FIG. 6A depicts the antenna 12 and the circuitry for amplifier 26a, highpass filter 28a, low pass filter 28b, amplifier 26b, modulator 30,oscillator 32 and high pass filter 34 (an LM356 operational amplifier isreferenced in the discussion of FIG. 5 and a pair of TL072 operationalamplifiers are identified in FIG. 6A). FIG. 6B depicts the circuitry foroscillator 16. FIG. 6C depicts the circuitry for amplifier 46 (notspecifically discussed in regard to FIG. 5, but shown in dashed lines onFIG. 5 since present in FIG. 6C), product detector 14, filter 44 andamplifier 36 (discussion of FIG. 5 indicates a LM380 and FIG. 6Cidentifies LM 384). FIG. 6D depicts the circuitry for the voltage leveldetector 22, rate meter 40 and pattern detector 42.

EXAMPLES Example 1

In this example, an apparatus according to that depicted in FIGS. 6A-6Dwas tested to determine the accuracy of the apparatus relative to knownsubsurface hydrocarbons. The Channing Gas Field over the MorrowFormations in eastern Colorado was selected as the site for the test. Asthe apparatus swept through the frequencies of passive electromagneticfields emanating from the Earth, tuned field activity react todiscontinuities caused by subsurface hydrocarbons and were expressed asa spike or series of spikes on the recorder opposite a given frequency.The frequency was related to depth using FIG. 2. The producing formationin this field is located at about 4600 feet below the Earth's surface.The equipment easily fitted into a small carrying case which readily fitin the rear of a sports utility vehicle.

At total of fourteen (14) readings at different locations were taken,but two were invalid because they were taken outside the preferred timewindow of the morning to early afternoon or because of atmosphericdisturbances such as thunderstorms. Eight (8) of the remaining readingswere over the oil bearing formation and four (4) outside the boundariesof the formation. Of the eight (8), six (6) readings gave positiveindications of the presence of hydrocarbons at the depth of interest.This represents a 75% success rating for detecting hydrocarbon bearingformations.

One (1) of the negative readings over the field was next to the Springer1-33 well. It is believed that this negative reading resulted due to thethin formation which is about 500 feet thick in this area and the effectof a nearby producing well had depleted the formation at this point. Itshould be noted that partially depleted fields in Texas such as theDelhi field and the Katy field did give good responses when surveyed bythis apparatus, but these fields produce from a thicker hydrocarboninterval. Similar false readings may occur in formation zones which havebeen water flooded, particularly if the zones are relatively thin.

One (1) of the positive points was repeated at a later time. These tworeadings were very similar and demonstrated the good repeatabilityachieved by the apparatus.

Of particular interest were the four (4) reading taken outside theformation boundaries. All of these reading gave negative indications forthe presence of hydrocarbons. Accordingly, the apparatus achieved a 100%success rating in determining the absence of hydrocarbons insubterranean formations. Three other fields in the Morrow Formation weretested outside the boundary of hydrocarbon bearing areas therein andachieved a success rating of about 93%. These results makes theapparatus quite valuable as a prospecting tool in wildcat areas, atleast to rule out areas where multiple readings are taken and asubstantial portion thereof provide negative indications for thepresence of hydrocarbons.

Example 2

In the example, the apparatus used in Example 1 was used to prospect forgold bearings formations. An area of interest was surveyed. Based ofsubsurface samples, the apparatus with multiple readings methodicallylocated about the area being prospected successfully located a largegold bearing deposit and provided the thickness and boundaries of thevein and angle of inclination thereof.

What is claimed is:
 1. An apparatus for passive geophysical prospectingbased upon detection of discontinuities associated with low frequencyelectromagnetic fields, the apparatus comprising:an antenna to pick upan extremely low frequency signal emanating from the Earth's surface,wherein the signal contains a range of frequencies and wherein each ofthe frequencies contained in the signal corresponds to a certain depthin the Earth and wherein the antenna generates a received signalcorresponding to the signal emanating from the Earth's surface; anamplifier and a first filter which in combination amplify and filter thereceived signal and generate an amplified, filtered signal; a modulatorfor modulating the amplified, filtered signal onto a carrier wave andgenerating a modulated signal having the carrier wave, an upper sideband and a lower sideband; a second filter for eliminating one of thesidebands; an oscillator which generates a tuning frequency, wherein theoscillator is capable of generating and sweeping through the frequenciescontained in the remaining sideband; and a product detector whichreceives the remaining sideband and multiplies it with the tuningfrequency generated by the oscillator generating a tuned signal, thetuned signal containing information regarding subterranean geologicformations at a certain depth corresponding to the tuning frequency. 2.A method for passive geophysical prospecting, the methodcomprising:receiving a signal emanating from the Earth's surface with anantenna; generating a received signal corresponding to the signalemanating from the Earth's surface, the received signal containing arange of frequencies; generating a tuning frequency; sweeping the tuningfrequency through at least the range of frequencies contained in thereceived signal; multiplying the received signal and the tuningfrequency to generate a product signal; synchronously tuning the productsignal over the range of frequencies contained in the received signal;and generating a tuned signal containing tuned information.
 3. Themethod of claim 2, further comprising:converting the tuned informationto pulse information representative thereof.
 4. The method of claim 3,wherein the converting step comprises comparing the tuned information toa reference point and generating pulse information relative to thereference point.
 5. A method for passive geophysical prospecting, themethod comprising:receiving a signal emanating from the Earth's surfacewith an antenna; generating a received signal corresponding to thesignal emanating from the Earth's surface; amplifying the receivedsignal to generate an amplified signal; generating a carrier wave usingan oscillator; modulating the carrier wave with the amplified signalgenerating a modulated signal having the carrier wave, an upper sidebandand a lower sideband; optionally canceling the carrier wave from themodulated signal generating an output signal having the upper and lowersidebands; and eliminating one of the sidebands and generating a singlesideband signal.
 6. The method of claim 5, further comprising:generatinga tuning frequency; sweeping the tuning frequency through at least therange of frequencies contained in the single sideband signal;multiplying the single sideband signal and the tuning frequency togenerate a product signal; synchronously tuning the product signal atleast over the range of frequencies contained in the single sidebandsignal; and generating a tuned signal for each tuning frequency, each ofthe tuned signals having tuned information relating to a particulardepth in the Earth's surface.